Composition and biophysical properties of lipids in Xenorhabdus nematophilus and Photorhabdus luminescens, symbiotic bacteria associated with entomopathogenic nematodes

Primary and secondary forms of Photorhabdus luminescens Hm and Xenorhabdus nematophilus N2-4 were grown at 18 and 28 degrees C for 24 to 96 h, and we made determinations of the fatty-acid compositions of total lipids and of the fluidity measured by 5-doxyl-stearic acid embedded in liposomes made from total lipids, The levels of the unsaturated fatty acids 16:1 and 18:1 (those with chain lengths of 16 or 18 and one double bond) generally were higher in primary-phase variants of P. luminescens grown at 18 degrees C than in those grown at 28 degrees C. Prolonged culture at 18 degrees C caused the level of 18:1 to fall and reach that observed at 28 degrees C, The ratio of saturated to unsaturated fatty acids rose with prolonged culture times in variants of each species at both phases, When grown at 18 degrees C, the proportion of 16:1 in X. nematophilus was lower than in P. luminescens; the patterns of temperature-induced changes were similar in these species, X. nematophilus contained a greater percentage of short-chain fatty acids (i.e., with chain lengths of <14.0) than P. luminescens. Lipid liposomes from primary and secondary cultures of both bacterial species grown at 18 degrees C were more ordered (i.e., less fluid) than those grown at 28 degrees C, This result suggests the surprising absence of homeoviscous adaptation of membranes to temperature, Also, liposomes from primary cultures were more ordered than those from secondary cultures and membranes from primary cultures of P. luminescens were more ordered at both culture temperatures than membranes from X. nematophilus. The biological significance of the effect of growth conditions on membrane biophysical properties in these bacteria is discussed.